A see-through head-mounted display (HMD) can provide transparent display area within a user's field of view in which a user can view both physical objects in the user's surroundings and virtual objects on the display. Some HMDs can provide “augmented reality” functionality by overlaying physical objects viewed by a user with digital content (such as text, pictures, and/or video) associated with the physical objects, or associated with the user's location and/or context, for example. Input to such HMDs are typically limited to buttons, a touch pad, or other simple input devices. These input devices can be bulky and inefficient.